In recent years a MIMO (Multi-Input Multi-Output) technique has been applied to radio communication systems, such as WiMAX (Worldwide Interoperability for Microwave Access) and 3GPP(3rd Generation Partnership Project)-LTE(Long Term Evolution), and has become an important technique for realizing high-speed transmission.
There are various techniques regarding MIMO demodulation. Typical techniques are MLD (Maximum Likelihood Detection), MMSE (Minimum Means Square Error), ZF (Zero Forcing), and the like. With the MLD a receiver calculates, for example, a metric for replicas of a plurality of transmitted signal points which are likely to be transmitted from a transmitter and a received signal point. The receiver then makes a search for a replica for which a calculated metric is the smallest. By doing so, the receiver estimates a transmitted signal point transmitted by the transmitter.
The MLD gives very high receiving performance compared with the MMSE or the ZF. The MLD can realize a low error rate in the same environment. On the other hand, a processing amount in the MLD is larger than a processing amount in the MMSE or the ZF. Circuit scale for the MLD is larger than circuit scale for the MMSE or the ZF. In addition, power consumption in the MLD is higher than power consumption in the MMSE or the ZF.
Many methods for reducing a processing amount in the MLD are devised. Methods based on QRM-MLD, symbol ranking, and the like are proposed. With these methods a search is not made for all replicas. On the basis of a step-by-step algorithm for narrowing down candidate replicas, the number of replicas to be searched for is reduced to reduce a processing amount.
Usually MLD receivers use squared Euclidean distance as a metric for many replicas and a received signal point. The reason for this is as follows. When a transmitted signal point is transmitted, first an MLD receiver makes a search for a transmitted signal point which maximizes the probability of a received signal point it receives. The probability of the transition from a transmitted signal point to a received signal point depends on squared Euclidean distance between a replica and the received signal point. That is to say, making a search for a replica of a transmitted signal point for which the probability is the highest is equivalent to making a search for a replica for which squared Euclidean distance is the shortest.
In the past, the following radio communication apparatus was provided. When radio communication is performed by the use of the MIMO technique, good characteristics can be realized. Furthermore, circuit scale and the number of operations are realistic (see, for example, Japanese Laid-open Patent Publication No. 2005-217506).
However, calculating squared Euclidean distance includes a square calculation and therefore needs a multiplier. In order to make a search for many replicas, a large number of multiplication processes are needed. This increases circuit scale and power consumption.